distinct locations,
which may or may not belong to the same organization.WAN
topologies use both LAN add enterprise-wide topologies as building
blocks, butadd more complexity
because of the distance they must cover, the larger number of usersthey serve, and the heavy traffic they often
handle. For example, although a simple ringtopology
may suffice for a small office with 10 users, it does not scale well and
thereforecannot serve 1000 users.
The particular WAN topology you choose will depend onthe
number of sites you must connect, the distance between the sites, and
any existinginfrastructure.

Peer-to-Peer

A WAN with single interconnection
points for each location is arranged in a peer-to-peer topology. A WAN
peer-to-peer topology is similar to peer-to-peer communications on a LAN
in that each site depends on every other site in the network to transmit
and receive its traffic. However, the peer-to-peer LANs use computers
with shared access to one cable, whereas the WAN peer-to-peer topology
uses different locations, each one connected to another one through
dedicated circuits.

The WAN peer-to-peer topology is
often the best option for organizations with only a few sites and the
capability to use dedicated circuits--that is, continuously available
communications channels between two access points that are leased from a
telecommunications provider, such as an ISP.

Ring

In a ring WAN topology, each site is
connected to two other sites so that the entire WAN forms a ring
pattern. This architecture is similar to the ring LAN topology, except
that a ring WAN topology connects locations rather than local nodes. The
advantages of a ring WAN over a peer-to-peer WAN are twofold: a single
cable problem will not affect the entire network, and routers at any
site can redirect data to another route if one route becomes too busy.
On the other hand, expanding a peer-to-peer WAN because it requires at
least one additional link. For those reasons, WANs that use the ring
topology are only practical for connecting fewer than four or five
locations.

Star

The star WAN topology mimics the arrangement of a star
LAN. A single site acts as the central connection point for several
other points. This arrangement provides separate routes for data between
any two sites. As a result, star WANs are more reliable than the
peer-to-peer or ring WANs. As a general rule, reliability increases with
the number of potential routes data can follow. Another advantage of a
star WAN is that when all of its dedicated circuits are functioning, a
star WAN provides shorter data paths between any two sites.

Mesh

Like an enterprise-wide mesh, a mesh WAN topology
incorporates many directly interconnected nodes--in this case,
geographical locatinos. Because every site is interconnected, data can
travel directly from its orgin to its destination. If one connection
suffers a problem, routers can redirect data easily and quickly. Mesh
WANs are the most fault-tolerant type of WAN configuration because they
provide multiple routes for data to follow between any two points.

One drawback to a mesh WAN is the cost; connecting every
node on a network to every other entails leasing a large number of
dedicated circuits. With larger WANs, the expense can become enormous.
To reduce costs, you might choose to implement a partial mesh, in which
critical WAN nodes are directly interconnected and secondary nodes are
connected through star or ring topologies. Partial-mesh WANs are more
practical, and therefore more common in today's business world, than
full-mesh WANs.

Tiered

Tiered WAN topologies are similar to the hierarchical
hybrid topologies used with LANs. In a tiered WAN topology, WAN sites
connected in a star or ring formations are interconnected at different
levels, with the interconnection points being organized into layers.

Variations on this topology abound. Indeed, flexibility
makes the tiered approach quite practical. A network architect can
determine the best placement of top-level routers based on traffic patterns
or critical data paths. In addition, tiered systems allow for easy
expansion and inclusion of redundant links to support growth. On the
other hand, their enormous flexibility means that creation of tiered
WANs requires careful consideration of geography, usage patterns, and
growth potential.